1. Field of the Invention
The present invention relates to an austenitic stainless steel that has good formability and is less susceptible to cracking during forming, and also relates to a method of manufacturing thereof.
2. Description of Related Art
Austenitic stainless steel has been applied to various uses in broader industrial fields, e.g. parts of motor vehicles and electronic parts, due to its excellent formability compared with other types. As development of usage, a demand for provision of cheaper material becomes stronger and stronger.
In order to meet with such the demand, the inventors have continued investigation on material design directed to soft austenitic stainless steel, which is scarcely work-hardened, while saving addition of expensive Ni content. As results of the investigation, the inventors proposed new austenitic stainless steel resistant to work-hardening and good of hot-workability, as disclosed in JP 9-263905 A1. The proposed austenitic stainless steel contains austenite formers such as Cu and Mn at proper levels, while hardening elements such as C and N contents are controlled to lower levels.
Parts or members for motor vehicles and electronic devices are designed to a small size or lightened in these days. The trend of small-sizing or lightening needs provision of a steel sheet, which is good of formability capable of forming to an objective shape with a heavy drawing ratio but thinner compared with a conventional steel sheet. Due to severe forming conditions, nonmetallic inclusions put significant influences on occurrence of cracking during forming. Typical inclusion, which promotes occurrence and development of cracking, is galaxite (MnO—Al2O3) in a high-Mn steel sheet.
FIG. 1 is a MnO—Al2O3 phase diagram (reported by Oelsen, W. G. Heynert, Arch. Eisenhüttenwes, 26(1955), p. 567). Galaxite, which is generated at 1720° C., is hard inclusion. Once galaxite is precipitated in a steel matrix at a steel-making or casting step, it is not deformed by hot-rolling or cold-rolling. Consequently, galaxite of several tens μm in size, which is equal to a size of galaxite generated at the steel-making or casting step, remains as such in a steel sheet. A steel sheet for use as a part of an electronic device or the like is of 0.1-0.5 mm in thickness. As decrease of thickness of a steel sheet, the size of galaxite is not neglected but accelerates initiation of cracking during forming. Even when cracking is suppressed, galaxite acts as a starting point to generate scratches on a surface of a product. If a surface of a product is damaged by scratches, the product cannot be offered to the market.